Donald Trump has spent the past two days bashing Marco Rubio with these insults as a way to send a message: Criticize the big dog and get bitten.

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On Thursday, Rubio barked back at Trump, calling him “a very touchy and insecure guy.” Rubio also attacked Trump at his points of pride: his stage presence and his poll numbers.

“He had a really bad debate performance last week,” Rubio told Kentucky Sports Radio. “He takes shots at everybody that gets anywhere close to him, in terms of a poll, or anytime he hits a rough spot that’s what he does.”

Hours later, Rubio made sure his criticisms were broadcast to a national audience during a Thursday interview on Fox News' "The O'Reilly Factor." Trump was supposed to be on the influential primetime show that night, but he was canceled in a dispute. Rubio jumped at the opportunity to take Trump's spot and bash him.

For Trump, it presents a new target and a fresh round of headlines. The billionaire businessman had slugged it out for weeks with Jeb Bush after taking on Scott Walker while taking potshots at almost every other candidate. Trump began attacking Rubio in force after the Florida senator leveled some of his most direct criticisms when he said Tuesday that the Republican front-runner hasn’t “met the threshold” to be president due to his relative lack of foreign-policy knowledge.

For Rubio, it's a gamble. It ends a long-term strategy of trying to ignore slugging it out with Trump. Rubio wants to avoid fighting on Trump’s terms. Rubio is a skill puncher. Trump’s a brawler. Polling in the middle of the pack, Rubio has been running a low-key campaign that has allowed him to pick friendly media outlets for interviews and has kept him largely out of the spotlight of a more critical mainstream media.

“I've made a decision here with Donald Trump, you know, if I comment on everything he says, my whole campaign will be consumed by it. That's all I'll do all day,” Rubio said in August on NBC's "Meet the Press." “We'll let him answer for what he says and so forth. At this point, I mean, we've got to focus on our message. Otherwise, my whole campaign will be what — how do you feel about what Donald Trump said about something. He says something every day.”

But since then, Rubio has taken an increasing number of veiled shots at Trump because it plays to what Rubio’s campaign sees as one of his strengths: foreign policy. As a member of the Senate Foreign Relations Committee, Rubio wants to talk about Iran, Vladimir Putin and Cuba. The front-runner Trump is a perfect foil. That creates conflict that produces free media coverage, but it also draws Rubio into a direct confrontation with Trump, whom he has largely avoided criticizing in the same way that, say, Bush has.

Also, Rubio’s criticisms aren’t part of a plan to make the contest a two-way race with Trump — a Bush tactic. Instead, Rubio’s campaign strategy revolves around waiting for the right opportunity to make news. Unlike Bush, who isn’t comfortable personally attacking another candidate, Rubio has fewer qualms about trading barbs and belittling an opponent — a tactic he honed in his 2010 Senate race against Gov. Charlie Crist (a Republican defector who was supported by Trump, Rubio noted during the first presidential debate).

And then there's Carly Fiorina. She proved last week that Trump's not invincible, landing a solid blow during the CNN debate when asked about his disparaging "look at that face" comment. She showed others that the counter-counter-punch can be pretty sweet, as shown by her surge in the polls.

Rubio, whose poll numbers also ticked up after the debate, also traded a few verbal jabs with Trump on stage. Rubio seemingly discerned that the best way to get at Trump was to fluster him, by trying to make him look small, foolish and feckless. So Rubio made sure Thursday to call Trump “insecure” and to reference the slide in the poll numbers that the front-runner likes to boast about.

And while polls show that Trump is still clearly the top dog in the GOP race, most surveys also show he’s its most polarizing figure and that Rubio is among Republicans’ most-liked, slightly trailing Fiorina and best-liked Ben Carson.

On Tuesday, after weeks of refusing to name Trump, the low-intensity conflict between the two came to a head when Fox NewsX host Bret Baier asked Rubio a direct question: “Is Donald Trump qualified to be president?”

Rubio gave a long, roundabout answer. Baier pressed: “Is that a no?”

“As of now, I haven’t heard him talk seriously about national security. Hopefully that will change,” Rubio said, giving another long explanation about the importance of foreign policy.

Rubio then closed with a shot at Trump.

“So far he hasn’t met the threshold in my mind,” Rubio said. Without a pause, he then partially chuckled: “but there’s still time, I suppose.”

Gauntlet thrown. Game on. Fight picked.

In Trump’s world, people who are nice to him aren’t savaged. People who criticize him directly get attacked.

“When Marco Rubio brought the fight to Donald Trump, Donald Trump responded in kind. And harder,” said Trump campaign manager Corey Lewandowski on Thursday. “He’s a counterpuncher. Trump never throws the first punch. But when he’s attacked, he’s a counterpuncher. I don’t think Trump starts the fights. Trump responds to those who fight with him.”

Trump has said as much on the campaign trail. When a Walker fundraiser criticized him in July, Trump sounded gleeful. “Finally, I can attack,” Trump said. “Finally.”

And so far, Trump's strategy appears to have worked. Every candidate whom Trump has mocked at length has stalled or fallen in the polls: Walker, Rick Perry and Bush. Trump remains far and away the GOP electorate’s favorite candidate right now, pulling about a quarter of the vote. Rubio barely cracks 9 percent in most polls and is running in fifth place, according to the Real Clear Politics average of Republican presidential polls.

From Tuesday on, Trump let Rubio have it, criticizing everything from his finances, to his hair, to his relationship with Jeb Bush, Rubio’s friend and former mentor who’s now his opponent.

“Rubio, I’ve never seen a young guy sweat that much,” Trump said in South Carolina during a speech where he talked about how hot the debate stage during last week’s marathon CNN debate. “He’s drinking water, water, water, I never saw anything like this with him with the water.”

For years, Rubio was dogged by accounting troubles connected to his political committees. Caught up in the heyday of the real estate boom, he wound up carrying three mortgages at one point. And he was criticized for ringing up personal charges on a Republican Party of Florida American Express card. Only recently, after two successful book deals, has Rubio put his finances in order.

Then Trump got personal. He said Rubio is “disloyal” by running against Bush.

“He’s overly ambitious, too young, and I have better hair than he does,” Trump said on Wednesday. The last line is a two-fer: Trump loves to boast about his hair and the 44-year-old Rubio is self-conscious about going bald early.

Trump again pointed out that Rubio “has the worst attendance rating in the U.S. Senate. He's got the No. 1 worst attendance record.” On Thursday, Trump called Rubio “a kid.”

On Thursday night, Rubio got the last word, for now, by appearing on O'Reilly. Asked about Walker's call to stop Trump, Rubio said the GOP needed "someone who is a serious person, someone who is serious about the future of our country."

Pressed by O'Reilly on whether he meant Trump, Rubio repeated his criticisms of the real estate mogul. "I think when the debate turned to policy, he stood still," Rubio said. "That's why he had a bad debate and acting the way he is and continuing to offend anyone who criticizes him." Rubio repeated that Trump "is thin-skinned and sensitive to criticism. ... He can't have a conversation about policy because quite frankly he doesn't know anything about policy.”

Rubio’s campaign wouldn’t discuss the candidate’s change of heart or tactics. But those who know him privately say that the candidate is being both strategic and prideful.

“Marco’s Cuban,” a fellow Cuban-American Republican from Miami said. “And at a certain point, you don’t take shit from nobody. The old man’s lucky Marco doesn’t just whip his ass.”

The root of the conflict started after Trump appeared on conservative commentator Hugh Hewitt’s show on Sept. 3 and had difficulty discussing Hamas, Hezbollah and the names of accused terrorist leaders. Trump said he was the victim of “gotcha” questions.

Rubio didn’t issue a statement or try to criticize Trump. Instead, the news media — and the opportunity — came to him when CNN a day later asked about Trump’s interview. Rubio said Trump’s comments were “very concerning.” Avoiding naming Trump, Rubio said Hewitt's queries weren't “gotcha” questions.

The simmering conflict started to intensify at last week’s debate, when CNN moderator Jake Tapper asked Trump about Rubio’s comments. Trump first said there was a “misunderstanding” in his interview with Hewitt and he promised that, as president, he would hire the right people.

“Well, you have to understand, I am not sitting in the United States Senate with, by the way, the worst voting record there is today,” Trump said, a reference to Rubio’s poor attendance record in the Senate. “No. 1, I am not sitting in the United States Senate. I'm a businessman doing business transactions.”

When it was his turn, Rubio articulated his foreign policy vision and the threats the United States faces from Iran, Russia and North Korea. Applause followed. Asked whether he had knowledge Trump didn’t have, Rubio wouldn’t say.

“You should ask him questions in detail about the foreign policy issues our president will confront, because you had better be able to lead our country on the first day,” Rubio told moderator Jake Tapper. “A president better be up-to-date on those issues on his first day in office, on her first day in office.”

More applause for Rubio.

Punch. Counter-punch.

This post has been updated to reflect Carson and Fiorina’s relative popularity in the GOP primary.